Air-void system in concrete: CT-based characterization and modeling for predicting microstructural development

• Applicant: Professor Dr.-Ing. Wolfgang Breit
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 576890251

Air voids are introduced into the fresh concrete by the mixing process and, depending on the exposure of a concrete component, must be stabilized by the addition of air entraining agents. Within the concrete matrix, they serve as an escape space to relieve the pressure generated during freezing of the pore fluid. In addition, they contribute to the reduction of capillary water absorption by interrupting the capillary pore system in the hardened concrete. This makes them a determining factor in the durability design of structural elements subject to freeze-thaw with and without de-icing salt attack. The total volume of air voids can be determined in fresh concrete according to DIN EN 12350-7 using the pressure equalization method. Moreover, the size of the voids and their distribution have a significant influence on the expected durability. These parameters can be determined for hardened concrete using the linear traverse method according to DIN EN 480-11. However, since the characterization of the air void system can be carried out at the earliest seven days after production, it is necessary to research methods that allow an early investigation on fresh concrete or mortar on the condition that results apply to hardened concrete properties with sufficient accuracy. Within the scope of this project, the physical and thermodynamic compression and solubility processes are analysed using computed tomography (CT) and numerical modelling of the air void system. For this purpose, fresh concrete specimens are prepared in different compositions and subjected to a predefined compressive load during CT analysis. The fresh concrete compositions differ in the amount and type of air entraining agent added to include the influence of the differing surface stresses and the resulting changes in solubility. The data obtained is then used to characterize the air entrained structure of the fresh concrete, and the transferability to the state of the pore system in hardened concrete. The aim is, on the one hand, to develop a model that represents the processes taking place. On the other hand, the link between the determined solubility rate as a function of the acting pressure and the existing pore sizes is analysed. This forms the basis for the specification of existing measurement methods and the derivation of the size distribution of entrained air voids in fresh concrete. The findings obtained in the first part form the necessary requirement for a subsequent, application-oriented follow-up project (part 2). In this, the developed models will be transferred to a broader range of practically relevant concrete compositions and validated with respect to their predictive capability for durability properties.

• DFG Programme: Research Grants